1. Field of the Invention
The present invention relates to projection lenses and projection systems, and, more particularly, to projection lenses and systems that provide improved use of the total light energy emitted by an illumination subsystem.
2. Description of Related Art
Light projection is used to display images on large surfaces, such as large computer displays or television screens. In front projection systems, an image beam is projected from an image source onto the front side of a reflection-type, angle transforming screen, which reflects the light toward a viewer positioned in front of the screen. In rear projection systems, the image beam is projected onto the rear side of a transmission-type, angle transforming screen and transmitted toward a viewer located in front of the screen.
In single exit pupil projection systems, three primary color images are projected through the same lens to form a full color image. These systems avoid color shift in the projected image and color mixing or combining need not be performed by their screen as in a three lens system. Single exit pupil systems may be either of the transmissive variety or of the reflective variety. Additional information about projection lenses and systems can be found in U.S. Pat. No. 5,218,480, issued to Moskovitch, entitled "Retrofocus Wide Angle Lenses," incorporated by reference herein in its entirety.
Several considerations stand out for such projection systems. One item is the efficient use of the light energy output of an illumination subsystem in a projection system. Matching the illumination subsystem with imagers (e.g., a liquid crystal display (LCD) or spatial light modulator (SLM)) in the projection system to obtain a bright, uniformly illuminated image is important. Etendue considerations have not been particularly emphasized in previous projection system designs. Examples of the type of light sources in illumination subsystems, amongst others, for which efficiency can matter include metal-halide lamps and those described in U.S. Pat. Nos. 5,404,076 and 5,606,220, issued to Dolan et al., entitled "Lamp Including Sulfur" and "Visible Lamp Including Selenium or Sulfur," respectively, and in U.S. Pat. No. Re. 34,492, issued to Roberts, entitled "Combination Lamp and Integrating Sphere For Efficiently Coupling Radiant Energy From A Gas Discharge Into A Lightguide." U.S. Pat. Nos. 5,404,076, 5,606,220, and Re. 34,492 are incorporated by reference herein in their entirety. Other examples include lamps described in PCT Pat. application No. PCT/US97/10490, by MacLennan et al., published as WO 97/45858 on Dec. 4, 1997, also incorporated by reference herein in its entirety.
Another consideration is system size. For rear projection and computer screen applications, a small overall package size is desirable except perhaps for the screen. The physical size of individual components, such as lenses, filters, stops, etc., should be made relatively small while a large image size should be produced. Although a system may be small in size, however, its compactness may not necessarily be optimized. For instance, in projection systems employing three LCD imagers, one for each primary color, the distance between the projection lens and the imagers may have to be increased to accommodate field lenses required to better match the illumination subsystem and the imagers.
In some previous projection lenses, the filtering of image or imager illumination light has been of concern. A filter could be placed, for example, within an aperture stop of a projection lens. However, aperture stops have previously been disadvantageously positioned within the physical confines of one of the lenses or other elements making up the projection lens.
Thermal effects have been a concern when polymer materials, despite their generally good optical properties, are used to construct individual lens elements in projection lens systems. Aspheres, although useful in limiting lens aberrations and in reducing lens size, can reveal detrimental thermal effects with high power light when positively powered optical elements are constructed of these materials. Acrylic materials, for example, present a relatively large change in refractive index with temperature. A lens fashioned out of acrylic can, therefore, display an internal temperature change or gradient. A corresponding optical power change can result with high powered light, leading to performance deficiencies.
Other considerations in projection systems include the effects of dispersion in optical elements and manufacturing tolerances. Dispersion effects frequently appear in optical systems in which all three primary colors are transmitted through the same optical elements. Manufacturing tolerances can impact parts interchangeability. Manufacturing tolerances may result in performance variations that need to be addressed by appropriate means to ensure that production model projection lenses and systems will demonstrate similar performance.
The present invention is directed to improving projection lenses and systems. The present invention is also directed to overcoming or reducing one or more of the problems and deficiencies set forth above or other problems and deficiencies.